Energy Sqrt Research Articles

Elliptic Flow of Charm and Strange Hadrons in High-Multiplicity p+Pb Collisions at sqrt[s_{NN}]=8.16 TeV.

The elliptic azimuthal anisotropy coefficient (v_{2}) is measured for charm (D^{0}) and strange (K_{S}^{0}, Λ, Ξ^{-}, and Ω^{-}) hadrons, using a data sample of p+Pb collisions collected by the CMS experiment, at a nucleon-nucleon center-of-mass energy of sqrt[s_{NN}]=8.16 TeV. A significant positive v_{2} signal from long-range azimuthal correlations is observed for all particle species in high-multiplicity p+Pb collisions. The measurement represents the first observation of possible long-range collectivity for open heavy flavor hadrons in small systems. The results suggest that charm quarks have a smaller v_{2} than the lighter quarks, probably reflecting a weaker collective behavior. This effect is not seen in the larger PbPb collision system at sqrt[s_{NN}]=5.02 TeV, also presented.

Measurement of the Absolute Branching Fraction of the Inclusive Decay Λ_{c}^{+}→Λ+X.

Based on an e^{+}e^{-} collision data sample corresponding to an integrated luminosity of 567 pb^{-1} taken at the center-of-mass energy of sqrt[s]=4.6 GeV with the BESIII detector, we measure the absolute branching fraction of the inclusive decay Λ_{c}^{+}→Λ+X to be B(Λ_{c}^{+}→Λ+X)=(38.2_{-2.2}^{+2.8}±0.9)% using the double-tag method, where X refers to any possible final state particles. In addition, we search for direct CP violation in the charge asymmetry of this inclusive decay for the first time, and obtain A_{CP}≡[B(Λ_{c}^{+}→Λ+X)-B(Λ[over ¯]_{c}^{-}→Λ[over ¯]+X)]/[B(Λ_{c}^{+}→Λ+X)+B(Λ[over ¯]_{c}^{-}→Λ[over ¯]+X)]=(2.1_{-6.6}^{+7.0}±1.6)%, a statistically limited result with no evidence of CP violation.

Inclusive study of bottomonium production in association with an eta meson in e^+e^- annihilations near varUpsilon (5S)

We study bottomonium production in association with an eta meson in e^+e^- annihilations near the varUpsilon (5S), at a centre-of-mass energy of sqrt{s}=10.866 GeV. The results are based on the 121.4 fb^{-1} data sample collected by the Belle experiment at the asymmetric-energy KEKB collider. Only the eta meson is reconstructed and the missing-mass spectrum of eta candidates is investigated. We observe the e^+e^-rightarrow eta varUpsilon _J(1D) process and find evidence for the e^+e^-rightarrow eta varUpsilon (2S) process, while no significant signals of varUpsilon (1S), h_b(1P), nor h_b(2P) are found. Cross sections for the studied processes are reported.

Search for beyond the standard model Higgs bosons decaying into a mathrm{b}overline{mathrm{b}} pair in pp collisions at sqrt{s}=13 TeV

A search for Higgs bosons that decay into a bottom quark-antiquark pair and are accompanied by at least one additional bottom quark is performed with the CMS detector. The data analyzed were recorded in proton-proton collisions at a centre-of-mass energy of sqrt{s}=13 TeV at the LHC, corresponding to an integrated luminosity of 35.7 fb−1. The final state considered in this analysis is particularly sensitive to signatures of a Higgs sector beyond the standard model, as predicted in the generic class of two Higgs doublet models (2HDMs). No signal above the standard model background expectation is observed. Stringent upper limits on the cross section times branching fraction are set for Higgs bosons with masses up to 1300 GeV. The results are interpreted within several MSSM and 2HDM scenarios.

Measurement of Ds\xb1 production asymmetry in pp collisions at sqrt{s}=7 and 8 TeV

The inclusive Ds± production asymmetry is measured in pp collisions collected by the LHCb experiment at centre-of-mass energies of sqrt{s}=7 and 8 TeV. Promptly produced Ds± mesons are used, which decay as Ds± → ϕπ±, with ϕ → K+K−. The measurement is performed in bins of transverse momentum, pT, and rapidity, y, covering the range 2.5 < pT< 25.0 GeV/c and 2.0 < y < 4.5. No kinematic dependence is observed. Evidence of nonzero Ds± production asymmetry is found with a significance of 3.3 standard deviations.

Measurements of b-jet tagging efficiency with the ATLAS detector using toverline{t} events at sqrt{s}=13 TeV

The efficiency to identify jets containing b-hadrons (b-jets) is measured using a high purity sample of dileptonic top quark-antiquark pairs ( toverline{t} ) selected from the 36.1 fb−1 of data collected by the ATLAS detector in 2015 and 2016 from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy sqrt{s}=13 TeV. Two methods are used to extract the efficiency from toverline{t} events, a combinatorial likelihood approach and a tag-and-probe method. A boosted decision tree, not using b-tagging information, is used to select events in which two b-jets are present, which reduces the dominant uncertainty in the modelling of the flavour of the jets. The efficiency is extracted for jets in a transverse momentum range from 20 to 300 GeV, with data-to-simulation scale factors calculated by comparing the efficiency measured using collision data to that predicted by the simulation. The two methods give compatible results, and achieve a similar level of precision, measuring data-to-simulation scale factors close to unity with uncertainties ranging from 2% to 12% depending on the jet transverse momentum.

Search for new phenomena using the invariant mass distribution of same-flavour opposite-sign dilepton pairs in events with missing transverse momentum in sqrt{s}=13\xa0text {Te}text {V}pp collisions with the ATLAS detector

A search for new phenomena in final states containing an e^+e^- or mu ^+mu ^- pair, jets, and large missing transverse momentum is presented. This analysis makes use of proton–proton collision data with an integrated luminosity of 36.1~mathrm {fb}^{-1}, collected during 2015 and 2016 at a centre-of-mass energy sqrt{s} = 13~hbox {TeV} with the ATLAS detector at the Large Hadron Collider. The search targets the pair production of supersymmetric coloured particles (squarks or gluinos) and their decays into final states containing an e^+e^- or mu ^+mu ^- pair and the lightest neutralino (tilde{chi }_1^0) via one of two next-to-lightest neutralino (tilde{chi }_2^0) decay mechanisms: tilde{chi }_2^0 rightarrow Z tilde{chi }_1^0, where the Z boson decays leptonically leading to a peak in the dilepton invariant mass distribution around the Z boson mass; and tilde{chi }_2^0 rightarrow ell ^+ell ^- tilde{chi }_1^0 with no intermediate ell ^+ell ^- resonance, yielding a kinematic endpoint in the dilepton invariant mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted using simplified models, and exclude gluinos and squarks with masses as large as 1.85 and 1.3 text {Te}text {V} at 95% confidence level, respectively.

Search for flavour-changing neutral current top-quark decays t \u2192 qZ in proton-proton collisions at sqrt{s}=13 TeV with the ATLAS detector

A search for flavour-changing neutral-current processes in top-quark decays is presented. Data collected with the ATLAS detector from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of sqrt{s}=13 TeV, corresponding to an integrated luminosity of 36.1 fb−1, are analysed. The search is performed using top-quark pair events, with one top quark decaying through the t → qZ (q = u, c) flavour-changing neutral-current channel, and the other through the dominant Standard Model mode t → bW. Only Z boson decays into charged leptons and leptonic W boson decays are considered as signal. Consequently, the final-state topology is characterized by the presence of three isolated charged leptons (electrons or muons), at least two jets, one of the jets originating from a b-quark, and missing transverse momentum from the undetected neutrino. The data are consistent with Standard Model background contributions, and at 95% confidence level the search sets observed (expected) upper limits of 1.7 × 10−4 (2.4 × 10−4) on the t → uZ branching ratio and 2.4 × 10−4 (3.2 × 10−4) on the t → cZ branching ratio, constituting the most stringent limits to date.

Measurement of the inelastic proton-proton cross section at sqrt{s}=13 TeV

A measurement of the inelastic proton-proton cross section with the CMS detector at a center-of-mass energy of sqrt{s}=13 TeV is presented. The analysis is based on events with energy deposits in the forward calorimeters, which cover pseudorapidities of −6.6 < η < −3.0 and +3.0 < η < +5.2. An inelastic cross section of 68.6 ± 0.5(syst) ± 1.6(lumi) mb is obtained for events with MX> 4.1 GeV and/or MY> 13 GeV, where MX and MY are the masses of the diffractive dissociation systems at negative and positive pseudorapidities, respectively. The results are compared with those from other experiments as well as to predictions from high-energy hadron-hadron interaction models.

Measurement of the inclusive J/psi polarization at forward rapidity in pp collisions at mathbf {sqrt{s} = 8}\xa0TeV

We report on the measurement of the inclusive J/psi polarization parameters in pp collisions at a center of mass energy sqrt{s} = 8 TeV with the ALICE detector at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 1.23 pb^{-1}. J/psi resonances are reconstructed in their di-muon decay channel in the rapidity interval 2.5< y < 4.0 and over the transverse-momentum interval 2< p_mathrm{T} < 15 mathrm {GeV}/c. The three polarization parameters (lambda _theta , lambda _varphi , lambda _{theta varphi }) are measured as a function of p_mathrm{T} both in the helicity and Collins-Soper reference frames. The measured J/psi polarization parameters are found to be compatible with zero within uncertainties, contrary to expectations from all available predictions. The results are compared with the measurement in pp collisions at sqrt{s} = 7 TeV.

Observation of tt[over ¯]H Production.

The observation of Higgs boson production in association with a top quark-antiquark pair is reported, based on a combined analysis of proton-proton collision data at center-of-mass energies of sqrt[s]=7, 8, and 13TeV, corresponding to integrated luminosities of up to 5.1, 19.7, and 35.9 fb^{-1}, respectively. The data were collected with the CMS detector at the CERN LHC. The results of statistically independent searches for Higgs bosons produced in conjunction with a top quark-antiquark pair and decaying to pairs of W bosons, Z bosons, photons, τ leptons, or bottom quark jets are combined to maximize sensitivity. An excess of events is observed, with a significance of 5.2 standard deviations, over the expectation from the background-only hypothesis. The corresponding expected significance from the standard model for a Higgs boson mass of 125.09GeV is 4.2 standard deviations. The combined best fit signal strength normalized to the standard model prediction is 1.26_{-0.26}^{+0.31}.

Search for top-squark pair production in final states with one lepton, jets, and missing transverse momentum using 36 fb\u22121 of sqrt{s}=13 TeV pp collision data with the ATLAS detector

The results of a search for the direct pair production of top squarks, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, several energetic jets, and missing transverse momentum are reported. The analysis also targets spin-0 mediator models, where the mediator decays into a pair of dark-matter particles and is produced in association with a pair of top quarks. The search uses data from proton-proton collisions delivered by the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of sqrt{s}=13 TeV and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36 fb−1. A wide range of signal scenarios with different mass-splittings between the top squark, the lightest neutralino and possible intermediate supersymmetric particles are considered, including cases where the W bosons or the top quarks produced in the decay chain are off-shell. No significant excess over the Standard Model prediction is observed. The null results are used to set exclusion limits at 95% confidence level in several supersymmetry benchmark models. For pair-produced top-squarks decaying into top quarks, top-squark masses up to 940 GeV are excluded. Stringent exclusion limits are also derived for all other considered top-squark decay scenarios. For the spin-0 mediator models, upper limits are set on the visible cross-section.

Prompt and non-prompt hbox {J}/psi production and nuclear modification at mid-rapidity in p\u2013Pb collisions at mathbf{sqrt{{ s}_{text {NN}}}= 5.02}\xa0TeV

A measurement of beauty hadron production at mid-rapidity in proton-lead collisions at a nucleon–nucleon centre-of-mass energy sqrt{s_text {NN}}=5.02 TeV is presented. The semi-inclusive decay channel of beauty hadrons into hbox {J}/psi is considered, where the hbox {J}/psi mesons are reconstructed in the dielectron decay channel at mid-rapidity down to transverse momenta of 1.3 GeV/c. The hbox {b}bar{hbox {b}} production cross section at mid-rapidity, hbox {d}sigma _{hbox {b}bar{hbox {b}}}/hbox {d}y, and the total cross section extrapolated over full phase space, sigma _{text {b}bar{text {b}}}, are obtained. This measurement is combined with results on inclusive hbox {J}/psi production to determine the prompt hbox {J}/psi cross sections. The results in p–Pb collisions are then scaled to expectations from pp collisions at the same centre-of-mass energy to derive the nuclear modification factor R_{text {pPb}}, and compared to models to study possible nuclear modifications of the production induced by cold nuclear matter effects. R_{text {pPb}} is found to be smaller than unity at low p_{mathrm{T}} for both hbox {J}/psi coming from beauty hadron decays and prompt hbox {J}/psi .

Search for Higgs boson decays to beyond-the-Standard-Model light bosons in four-lepton events with the ATLAS detector at sqrt{s}=13 TeV

A search is conducted for a beyond-the-Standard-Model boson using events where a Higgs boson with mass 125 GeV decays to four leptons (ℓ = e or μ). This decay is presumed to occur via an intermediate state which contains one or two on-shell, promptly decaying bosons: H → ZX/XX → 4ℓ, where X is a new vector boson Zd or pseudoscalar a with mass between 1 and 60 GeV. The search uses pp collision data collected with the ATLAS detector at the LHC with an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy sqrt{s}=13 TeV. No significant excess of events above Standard Model background predictions is observed; therefore, upper limits at 95% confidence level are set on modelindependent fiducial cross-sections, and on the Higgs boson decay branching ratios to vector and pseudoscalar bosons in two benchmark models.

Search for supersymmetry in final states with missing transverse momentum and multiple b-jets in proton-proton collisions at sqrt{s}=13 TeV with the ATLAS detector

A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks into the lightest neutralino left({tilde{chi}}_1^0right) is reported. It uses LHC proton-proton collision data at a centre-of-mass energy sqrt{s}=13 TeV with an integrated luminosity of 36.1 fb−1 collected with the ATLAS detector in 2015 and 2016. The search is performed in events containing large missing transverse momentum and several energetic jets, at least three of which must be identified as originating from b-quarks. To increase the sensitivity, the sample is divided into subsamples based on the presence or absence of electrons or muons. No excess is found above the predicted background. For {tilde{chi}}_1^0 masses below approximately 300 GeV, gluino masses of less than 1.97 (1.92) TeV are excluded at 95% confidence level in simplified models involving the pair production of gluinos that decay via top (bottom) squarks. An interpretation of the limits in terms of the branching ratios of the gluinos into third-generation squarks is also provided. These results improve upon the exclusion limits obtained with the 3.2 fb−1 of data collected in 2015.

Measurement of Prompt D^{0} Meson Azimuthal Anisotropy in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The prompt D^{0} meson azimuthal anisotropy coefficients, v_{2} and v_{3}, are measured at midrapidity (|y|<1.0) in Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=5.02 TeV per nucleon pair with data collected by the CMS experiment. The measurement is performed in the transverse momentum (p_{T}) range of 1 to 40 GeV/c, for central and midcentral collisions. The v_{2} coefficient is found to be positive throughout the p_{T} range studied. The first measurement of the prompt D^{0} meson v_{3} coefficient is performed, and values up to 0.07 are observed for p_{T} around 4 GeV/c. Compared to measurements of charged particles, a similar p_{T} dependence, but smaller magnitude for p_{T}<6 GeV/c, is found for prompt D^{0} meson v_{2} and v_{3} coefficients. The results are consistent with the presence of collective motion of charm quarks at low p_{T} and a path length dependence of charm quark energy loss at high p_{T}, thereby providing new constraints on the theoretical description of the interactions between charm quarks and the quark-gluon plasma.

Jet properties in PbPb and pp collisions at sqrt{s_{mathrm{N};mathrm{N}}}=5.02 TeV

Modifications of the properties of jets in PbPb collisions, relative to those in pp collisions, are studied at a nucleon-nucleon center-of-mass energy of sqrt{s_{mathrm{N};mathrm{N}}}=5.02 TeV via correlations of charged particles with the jet axis in relative pseudorapidity (Δη), relative azimuth (Δϕ), and relative angular distance from the jet axis varDelta mathrm{r}=sqrt{{left(varDelta eta right)}^2+{left(varDelta phi right)}^2} . This analysis uses data collected with the CMS detector at the LHC, corresponding to integrated luminosities of 404 μb−1 and 27.4 pb−1 for PbPb and pp collisions, respectively. Charged particle number densities, jet fragmentation functions, and jet shapes are presented as a function of PbPb collision centrality and charged-particle track transverse momentum, providing a differential description of jet modifications due to interactions with the quark-gluon plasma.

Search for a new heavy gauge-boson resonance decaying into a lepton and missing transverse momentum in 36 fb^{-1} of pp collisions at sqrt{s} = 13\xa0TeV with the ATLAS experiment

The results of a search for new heavy W^prime bosons decaying to an electron or muon and a neutrino using proton–proton collision data at a centre-of-mass energy of sqrt{s}~=~13 TeV are presented. The dataset was collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 text{ fb }^{-1}. As no excess of events above the Standard Model prediction is observed, the results are used to set upper limits on the W^prime boson cross-section times branching ratio to an electron or muon and a neutrino as a function of the W^prime mass. Assuming a W^prime boson with the same couplings as the Standard Model W boson, W^prime masses below 5.1 TeV are excluded at the 95% confidence level.

Searches for heavy ZZ and ZW resonances in the \u2113\u2113qq and \u03bd\u03bdqq final states in pp collisions at sqrt{s}=13 TeV with the ATLAS detector

This paper reports searches for heavy resonances decaying into ZZ or ZW using data from proton-proton collisions at a centre-of-mass energy of sqrt{s}=13 TeV. The data, corresponding to an integrated luminosity of 36.1 fb−1, were recorded with the ATLAS detector in 2015 and 2016 at the Large Hadron Collider. The searches are performed in final states in which one Z boson decays into either a pair of light charged leptons (electrons and muons) or a pair of neutrinos, and the associated W boson or the other Z boson decays hadronically. No evidence of the production of heavy resonances is observed. Upper bounds on the production cross sections of heavy resonances times their decay branching ratios to ZZ or ZW are derived in the mass range 300-5000GeV within the context of Standard Model extensions with additional Higgs bosons, a heavy vector triplet or warped extra dimensions. Production through gluon-gluon fusion, Drell-Yan or vector-boson fusion are considered, depending on the assumed model.

Search for W W/W Z resonance production in \u2113\u03bdqq final states in pp collisions at sqrt{s}=13 TeV with the ATLAS detector

A search is conducted for new resonances decaying into a W W or W Z boson pair, where one W boson decays leptonically and the other W or Z boson decays hadronically. It is based on proton-proton collision data with an integrated luminosity of 36.1 fb−1 collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of sqrt{s}=13 TeV in 2015 and 2016. The search is sensitive to diboson resonance production via vector-boson fusion as well as quark-antiquark annihilation and gluon-gluon fusion mechanisms. No significant excess of events is observed with respect to the Standard Model backgrounds. Several benchmark models are used to interpret the results. Limits on the production cross section are set for a new narrow scalar resonance, a new heavy vector-boson and a spin-2 Kaluza-Klein graviton.